| 摘要 |
A metrology apparatus for determining bias and overlay errors in a substrate formed by a lithographic process includes an aperture between the objective lens and the image plane adapted to set the effective numerical aperture of the apparatus. The aperture is adjustable to vary the effective numerical aperture of the apparatus and the aperture may be non-circular, to individually vary the effective numerical aperture of the apparatus in horizontal and vertical directions. To determine bias and overlay error there is provided a target having an array of elements on the substrate, the array comprising a plurality of spaced, substantially parallel elements having a length and a width, the sum of the width of an element and the spacing of adjacent elements defining a pitch of the elements, edges of the elements being aligned along a line forming opposite array edges, the distance between array edges comprising the array width.
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